(1) Field of the Invention
The present invention relates to thermoplastic polyurethanes obtainable by reacting (a) isocyanates with (b1) polyesterdiols having a melting point greater than 150° C., preferably from 151 to 260° C., particularly preferably from 165 to 245° C., (b2) polyetherdiols and/or polyesterdiols, each having a melting point of less than 150° C., preferably from 0 to 149° C., and a molecular weight of from 501 to 8000 g/mol, and, if required, (c) diols having a molecular weight of from 62 to 500 g/mol. The present invention furthermore relates to a process for the preparation of thermoplastic polyurethanes and products containing the novel thermoplastic polyurethanes.
(2) Description of Related Art
Thermoplastic elastomers are based on a standard structural principle, regardless of their chemical composition. They are block copolymers in which hard blocks are connected to soft blocks in a polymer chain. Hard blocks are to be understood as meaning polymer segments whose softening temperature—glass transition temperature or crystallite melting point—is far above the temperature of use. Soft blocks are polymer segments having softening temperatures well below the temperature of use, preferably less than 0° C. The hard blocks form physical networks which can be reversibly cleaved during the thermoplastic processing and reformed on cooling.
Typical examples are styrene/butadiene block copolymers having hard polystyrene blocks (glass transition temperature about 105° C.) and soft polybutadiene blocks (glass transition temperature about −90° C.) or thermoplastic polyurethanes (TPU). The latter product group has, as a semicrystalline hard phase, the reaction product of an organic diisocyanate with a low molecular weight diol and, as an amorphous soft phase, the reaction product of an organic diisocyanate with a polyesterdiol or a polyetherdiol having molecular weights of, usually, from 500 to 5000 g/mol.
The solidification behavior of this semicrystalline polyurethane hard phase is however very variable and can easily be influenced, deteriorations generally occurring. For example, an increase in the index (ratio of moles of isocyanate to moles of OH-containing components) to the range of from 1.05 to 1.20 has a very adverse effect, as does the addition of other polymers. The conventional preparation processes, such as belt and extruder processes, also lead, with the same formulation, to TPU having substantially different crystallization behavior. However, in all processing methods, whether injection molding or extrusion, a constant and rapid solidification rate is a substantial factor influencing the uniform quality of the shaped articles, properties such as hardness, strength, rigidity and heat distortion resistance, and the cost-efficiency of the production.
A wide range of efforts has been made to compensate this disadvantageous behavior of the TPU. Apart from the addition of nucleating agents, e.g. finely divided talc, attempts have also been made to achieve improvements by adding other, rapidly crystallizing polymers. Thermoplastic, semicrystalline polyesters were particularly frequently used, among these preferably polybutylene terepthalate, owing to its melting range of from 220 to 230° C. which very well matches the customary TPU processing temperatures.
Thus, DE-A 26 46 647 describes the compounding of prepared, high molecular weight polyesters and high molecular weight TPU in single-screw or twin-screw extruders. EP-A 334 186 and DE-A 41 13 891 disclose the compounding of high molecular weight polyesters and TPU monomer components. DE-A 41 28 274 describes the addition of up to 5% of diisocyanate in excess for such processes. For an improvement in the compatibility, EP-A 656 397 describes the use of a TPU having an index greater than 1.16 and admixture with high molecular weight polyester. These processes lead to two-phase polymer mixtures, the polymer component, when present in amounts greater than 50%, having particle sizes of from 10 to 50 μm or <5 μm with the use of additional diisocyanate. Such molding materials are said to have higher strength, rigidity and heat distortion resistance than an unmodified TPU. On the other hand, the fact that a substantial reduction in the solidification rate occurs in particular on addition of diisocyanate in excess is very disadvantageous.
EP-A 102 115 and EP-A 846 712 describe the reaction of polyalkylene terephthalates with aliphatic polyesters to give block copolyester-esters, which in turn are then reacted with organic diisocyanates. The polycondensation of dimethyl terephthalate, butanediol and polyetherdiol and the subsequent reaction of the polyester with further polyetherdiol and diisocyanates to give a high molecular weight product are described in WO 99/51656. The long reaction times and the high temperatures, which easily lead to pronounced discoloration of the molding materials, are disadvantageous in all these processes.
DE-A 199 39 112 describes the degradation of hard thermoplastic polyurethanes with low molecular weight diols and the subsequent reaction with isocyanates for the preparation of flexible TPU.